We have shown that normal human B cells as well as EBV transformed B cell lines with accessory cell activities, produce IL 1-like factors. The normal B cell derived IL 1 is biochemically and antigenically similar to monocyte derived IL 1, but the IL 1 produced by the transformed B cell line shows some unique biochemical properties and is not inhibited by rabbit and human monocyte derived IL 1. Studies of the rate of production of IL 1 by monocytes reveal it to appear intracellularly within 30' after stimulation, and extracellularly with 60'. The m.w. of the predominant intracellular IL 1 activity is 23 kD, with a minor peak at 30 kD, whereas the intracellular IL 1 is largely (greater than 95%) associated with the cytosol fraction and only 1-2% is present in the membrane and particulate fraction of the cells. These data suggest IL 1 is rapidly processed from largely biologically inactive 30 kD precursor to an active 23 kD cell associated and 17 kd soluble forms. We have purified the soluble (pI 6.9, l7 kD) form of human IL 1 and obtained a partial amino acid sequence of the amino terminal end of the molecule. We have radiolabelled this IL 1 with 125I and demonstrated inhibitable binding sites on some human B cell lines. We have also demonstrated that IL 1 augments the in vitro tumoricidal activities both of human natural killer cells and monocytes and that IL 1 also is directly cytostatic and cytocidal for a limited number of tumor cell lines. These in vitro observations reinforce the hypothesis that IL 1 may be a BRM which promotes host antitumor defense mechanisms.